High power fiber lasers have become increasingly popular due to their high efficiency, simplicity and reliability. In addition, they may be easily ruggedized, due to their simple arrangement.
High power applications generally use a double clad fiber. This fiber comprises a core, usually doped with a lasing material such as rare earth ions or other, an inner cladding encircling the doped core, through which the pump power flows and is gradually absorbed in the doped core, and an outer cladding the inner cladding and forming a dielectric wave guide for the pump signal. The optical characteristics of the inner cladding closely match high power diode lasers, commonly used for solid-state laser pumping. Therefore, highly efficient pumping may be achieved by utilizing double clad fibers as a gain material.
One of the problems in double clad fibers, used for high power fiber laser applications, is the end pumping approach for injecting optical pump power. End pumping provides at most only two input ends for each fiber in the laser system, through which all the injected power enters the fiber. This physical limit constrains the number and type of pump sources that may be used to inject the optical power. In addition, when the double clad fiber is used as a power amplifier, end pumping prohibits simple injection of the signal to be amplified, and renders the coupling optics cumbersome and expensive.
Modern high power pumping techniques for commercial fiber lasers and amplifiers are usually based on end pumping by diode lasers. The common fibers used for fiber lasers applications are Yb3+ doped silica with tunable output between 980 nm-1200 nm (pumped by either 915 nm or 980 nm diodes), Er3+ doped silica for 1550 nm eye-safe and communication applications (pumped by either 980 nm or 1480 nm diodes), and Yb3+:Er3+ silica fibers used also for 1550 nm applications, but in the high power range, where the wide spread erbium doped fibers are not applicable. Other fiber lasers used mostly for 2 μm remote sensing and medical applications are Tm3+ doped and Ho3+:Tm3+ doped silica fibers.
The most commonly used fiber for marking, drilling and other industrial applications is the Yb3+ fiber, characterized by high efficiency and robustness. In addition, reliable and efficient pump diodes are available for this ion excitation, while its wide absorption band (25 nm) enables using pump diodes that do not need special cooling. The fiber's high efficiency and high surface-to-volume ratio enables cooling by air rather than cumbersome liquid cooling in solid-state lasers.
One of the main limitations today in using high power fiber lasers and amplifiers is, however, the pump coupling technique. Reference is now made to FIG. 1, which illustrates a prior art end coupling in a high power fiber amplifier. A high power diode 10 may pump optical power to a rare-earth doped double clad fiber 18 (e.g., Yb3+ doped fiber), through coupling optics 12 and an end-fiber coupling section 14. A seeder 16, such as a 1.064 μm diode, may inject low power signals to coupling section 14. Coupling section 14 may be coated for anti-reflection at the pump wavelength and may have high reflection at the signal wavelength. The double clad fiber 18 may be connected to output coupling optics 20.
However, the end pumping technique may limit coupling efficiency, lower the fiber laser system robustness, due to the complex optics alignment and tight tolerances required, and also increase the system cost, due to the expensive optics used. The problem becomes even more severe when high power fiber amplification is required. The complex alignment and tight tolerances, along with the high power flux at the fiber input end, render this configuration complex, inefficient, expensive and very sensitive to environmental changes.
Solutions have been proposed to these problems in the prior art. For example, U.S. Pat. No. 5,999,673 to Samartsev et al. describes a coupling between a multi-mode optical fiber pigtail and a double-clad optical fiber, that is, a fiber that includes an inner (single-mode or multi-mode) core with a diameter of few microns, a first cladding (multi-mode), and a second cladding. Samartsev et al. attempt to transfer multi-mode light source power to an optical fiber along a non-coaxial direction.
The coupling in Samartsev et al. comprises a tapered circular pump-guiding multi-mode fiber between the double clad fiber's inner cladding and the pump source. The pump-guiding fiber is tapered and then fused to the double clad fiber's inner clad, where the fusion region contains substantially the whole tapered region of the pump-guiding fiber, and nothing else. However, the divergence angle of the pump-guiding fiber, αs, and that of the multi mode inner cladding part of the double clad fiber, αf, has to satisfy the following relation:αf=k·αs
wherein k is a constant greater than 1.
There is an interest in using pump guiding fibers satisfying k<=1, since these pump guiding fibers can deliver more power than pump guiding fibers satisfying the k>1 condition, as in Samartsev et. al. Pump guiding fibers satisfying k<=1 have a higher numerical aperture than pump guiding fibers with k>1, and therefore, low brightness pump diode light with higher power can be efficiently coupled to these fibers, whereas with pump guiding fibers satisfying k>1, as in Samartsev et. al, the coupling efficiency is low.
Another disadvantage of Samartsev et al. is that when using fibers with k<=1, (that is, the divergence angles of the pump guiding fiber is higher or equal to that of the inner cladding of the double clad fiber) where the fusion region is only the tapered region of the pump-guiding fiber, than the pump coupling efficiency between the pump guiding fiber and the inner cladding of the double clad fiber is low and renders the Samartsev et al. method inefficient.